The present invention relates to an image forming apparatus equipped with a developer charging member for charging the developer remaining on an image bearing member, and is suitable for a cleaner-less image forming apparatus, that is, an image forming apparatus which does not have a dedicated cleaner. In particular, it relates to a cleaner-less image forming apparatus, in which the developer (toner) remaining on an image bearing member after the image transfer process is removed (recovered) from the image bearing member by the developing apparatus so that the recovered developer can be reused.
Heretofore, an electrophotographic image forming apparatus of a transfer type, such as a copying machine a printer, a facsimile, etc., comprises: a photoconductive member as an image bearing member which usually is in the form of a drum; a charging apparatus (charging process) for uniformly charging the photoconductive drum to predetermined polarity and potential level; an exposing apparatus (exposing process) as an information writing means for forming an electrostatic latent image on the charged photoconductive member; a developing apparatus (developing process) for visualizing the electrostatic latent image formed on the photoconductive member with the use of toner as developer; a transferring apparatus (transferring process) for transferring the toner image from the surface of the photoconductive drum onto transfer medium, for example, a piece of paper; a cleaning apparatus (cleaning process) for cleaning the surface of the photoconductive drum by removing the toner remaining, by a certain amount, on the surface of the photoconductive drum; a fixing apparatus (fixing process) for fixing the toner image on the transfer medium; and so forth. The photoconductive member is repeatedly subjected to an electrophotographic processes (charging, exposing, developing, transferring, and cleaning processes) to form images.
The toner remaining on the photoconductive drum after the transferring process is removed from the surface of the photoconductive drum by the cleaning apparatus, and collected as waste toner in the cleaning apparatus. From the standpoint of environmental preservation, effective utilization of natural resources, and so on, it is desired that waste toner such as the above described one is not generated.
Thus, there has been developed an image forming apparatus in which the untransferred residual toner, or the so-called waste toner collected in the cleaning apparatus, is returned to the developing apparatus to be reused.
There has also been developed a cleaner-less image forming apparatus which does not have a dedicated cleaning apparatus, and in which the untransferred residual toner, or the toner remaining on the photoconductive drum after the transferring process, is removed from the photoconductive drum by the developing apparatus to be reused, at the same time as an electrostatic latent image on the photoconductive drum is developed by the developing apparatus (developing/cleaning process).
The elimination of the dedicated cleaning system makes it possible to reduce image forming apparatus size and simplify an Image forming apparatus. Further, the lack of a dedicated cleaning member means that there is no rubbing of the surface of the photoconductive drum by the cleaning member, lengthening the service life of the photoconductive drum. In other words, the elimination of the dedicated cleaning system offers substantial merits.
The developing/cleaning process is a process in which the toner remaining on the photoconductive drum after the image transfer is recovered by the developing apparatus during the following developing process. More specifically, after the image transfer, the area of the photoconductive drum, from which the toner image has been transferred, is charged, and then, is exposed to form an electrostatic latent image thereon. Then, the untransferred residual toner on the portions of the peripheral surface of the photoconductive member (non-image portions), to which toner is not to be adhered, is recovered into the developing apparatus, by the fog prevention bias (difference Vback in potential level between DC voltage applied to developing apparatus, and the surface potential level of photoconductive drum. According to this method, the untransferred residual toner is recovered into the developing apparatus and is reused for developing electrostatic latent image in the following image formation cycles. In other words, no toner is wasted.
Therefore, a user does not need to be bothered by the waste toner.
Further, having no dedicated cleaner is advantageous from the standpoint of image forming apparatus size reduction. Since the untransferred residual toner on the photoconductive drum is recovered by the developing apparatus, it is desired that a reversal developing method, that is, a developing method in which the polarity to which the photoconductive drum is charged is the same as the normal polarity to which toner is charged, is employed.
However, if a cleaner-less image forming apparatus such as the above described one which recovers (removes) the transfer residual toner remaining on the photoconductive drum after image transfer, and reuse it, is such an image forming apparatus that employs a contact charging apparatus which charges the surface of the photoconductive member by making contact with the photoconductive member, the toner particles in the untransferred residual toner, the polarity of which have been made opposite to the normal polarity to which the toner becomes charged, adhere to the contact charging apparatus while the transfer residual toner on the photoconductive member passes the charging station, that is, the contact nip between the photoconductive member and contact charging apparatus, contaminating the contact charging apparatus beyond the tolerable range. As a result, the photoconductive member is unsatisfactorily charged.
More specifically, normally, the toner as developer contains a certain amount of toner particles, the polarity of which is opposite to the normal toner polarity, although the amount is relatively small. Further, some of the toner particles with the normal polarity are reversed in polarity, or reduced in the amount of charge, by the transfer bias, the electrical discharge from the recording medium separation, etc.
Thus, the untransferred residual toner contains the toner particles with the normal polarity, toner particles with the reverse polarity, and toner particles with a smaller amount of electrical charge Among these three types of toner particles, the toner particles with the reverse polarity and the toner particles with reduced electrical charge are likely to adhere to the contact charging apparatus while they are moving through the charging station, or the contact nip between the photoconductive drum and contact charging apparatus.
Further, in order to remove and recover the untransferred residual toner on the photoconductive drum (in order to clean the photoconductive drum) by the developing apparatus at the same time as a latent image on the photoconductive drum is developed by the developing apparatus, it is necessary that the toner particles in the untransferred residual toner on the photoconductive drum, which are being carried to the developing station through the charging station, are normal in polarity (for example, negative), and also that the amount of electrical charge they are holding is proper for them to be used by the developing apparatus to satisfactorily develop the electrostatic latent image on the photoconductive drum. The toner particles with the reverse polarity (for example, positive polarity) and the toner particles improper in the amount of electrical charge cannot be removed and recovered from the photoconductive drum by the developing apparatus, effecting unsatisfactory images.
An image defect traceable to the failed recovery of the positively charged toner particles by the developing apparatus is called a positive ghost, which is a problem peculiar to an image forming apparatus without a dedicated cleaning member. More specifically, without a dedicated cleaning member, the untransferred residual toner reaches the developing station past the charging member, while remaining distributed in the pattern of the electrostatic latent image. If the untransferred residual toner particles are satisfactorily removed, in the developing station, from the photoconductive drum by the developing apparatus while the next electrostatic latent image is developed by the developing apparatus, the pattern in which the untransferred residual toner particles are distributed is eliminated. However, if the residual toner particles fall to be satisfactorily removed, the pattern which the residual toner particles are distributed is not completely eliminated and appears across a transfer medium, overlapping with the following toner image, as the following toner image is transferred onto the transfer medium. As a result, the portions of the following toner image corresponding to the residual toner pattern appear darker; in other words, a ghost appears. Since this ghost is darker than the surrounding area, it is called a positive ghost.
The above described adhesion of the toner particles to the contact charging apparatus can be prevented by charging the untransferred residual toner, that is, a mixture of the toner particles with the normal polarity, toner particles with the reversal polarity, and toner particles with reduced electrical charge, with the use of means for controlling the electrical charge of the untransferred residual toner particles in polarity as well as amount, so that so that all the toner particles in the residual toner become normally charged, and uniform in the amount of electrical charge.
However, as the residual toner particles are charged by the toner charge controlling means in order to prevent them from adhering to the contact charging apparatus, the amount of the electrical charge of the residual toner particles becomes greater than the proper amount of electrical charge for the satisfactory development of the electrostatic latent image on the photoconductive drum, making it difficult for the residual toner particles to be removed and recovered by the developing apparatus in the developing station, at the same time as the developing process is carried out by the developing apparatus. As a result, some of the toner particles in the residual toner remain on the photoconductive drum and are transferred onto a recording medium, effecting image defects, as the following toner image is transferred onto the recording medium.
Further, in recent years, the user needs have diversified. As a result, the demand for an image forming apparatus capable of continuously forming images with a high printing ratio, such as photographic images, an image forming apparatus capable of forming color images with the use of a multilayer developing method or the like, and the like image forming apparatuses, has increased. In the case of such image forming apparatuses, a large amount of the residual toner is generated all at once, exacerbating the above described problems.
This problem can be solved by providing a cleaner-less image forming apparatus with a residual toner particle uniformizing means (first developer charging member) and a toner charge amount controlling means (second developer charging means), and applying predetermined DC voltages to the two means. The residual toner particle uniformizing means is a means for making uniform in polarity the transfer residual toner particles remaining on the photoconductive drum after the transfer of the toner image on the photoconductive drum, whereas the toner charge amount controlling means is a means for charging the residual toner particles on the photoconductive drum. In terms of the rotational direction of the photoconductive drum, the residual toner particle uniformizing means is positioned on the upstream side of the contact charging apparatus and on the downstream side of the transferring means, whereas the toner charge amount controlling means is positioned on the down stream side of the residual toner uniformizing means and on the upstream side of the contact charging apparatus. The details of this solution is disclosed in U.S. Pat. No. 6,421,512.
More concretely, the residual toner particles remaining on the photoconductive drum after the toner image transfer are uniformized by the residual toner uniformizing means, and then, the uniformized residual toner particles on the photoconductive drum are charged to the normal polarity by the toner charge amount controlling means. Thereafter, at the same time as the surface of the photoconductive drum is charged in the charging station by the contact charging apparatus, the residual toner particles are charged by the contact charging apparatus to the proper potential level for the toner particles to be removed and recovered from the photoconductive drum by the developing apparatus in the developing station at the same time as the developing process is carried out by the developing apparatus in the developing station. Then, the properly charged residual toner particles are recovered by the developing apparatus in the developing station.
To describe in more detail, the image forming apparatus is provided with two stationary brush, as the first (upstream) and second (upstream) developer charging members, which are disposed on the downstream side of the transferring means and on the upstream side of the charging means. To the first developer charging member, positive DC voltage (positive bias) is applied, whereas to the second developer charging member, negative DC voltage (negative bias) is applied. The negatively charged toner particles on the photoconductive member are absorbed by the first developer charging member, being thereby positively charged. As the amount of the negatively charged toner particles in the first developer charging member reaches the toner particle holding capacity of the first developer charging member, the toner particles in the first developer charging member are gradually expelled as positively charged toner particles, back onto the photoconductive member. Thus, all the residual toner particles on the area of the peripheral surface of the photoconductive member on the immediately downstream side of the first developer charging member have positive electrical charge. Then, all the charged residual toner particles on this area of the peripheral surface of the photoconductive member are efficiently charged to the negative polarity by the second developer charging member, since all the residual toner particles on this area have been positively charged by the first developer charging member. As a result, the residual toner particles are prevented from adhering to the charging means (charge roller).
Further, oscillatory voltage, more specifically, a combination of DC voltage and AC voltage, is applied to the charging means. Therefore, the electrical charge of the residual toner particles, which is relatively high in potential level after being charged by the second developer charging member, is removed by a certain amount by the charging means. As a result, the potential level of the residual toner particles reduces to the potential level (close to proper level for satisfactory development) at which the residual toner particles can be easily recovered by the developing apparatus, improving thereby the efficiency with which the residual toner particles are recovered by the developing apparatus.
However, the amount by which electrical charge is given to the residual toner particles remaining on the photoconductive drum after the toner image transfer significantly affected by the conditions of the environment in which an image forming apparatus is used, printing ratio, etc. Therefore, if the DC voltages applied to the first and second developer charging members are kept constant, the residual toner particles sometimes fails to be charged to the proper potential level for them to be removed and recovered by the developing apparatus. In such a case, the residual toner particles remaining on the photoconductive drum, that is, the toner particles which could not be removed and recovered by the developing apparatus, are transferred onto a transfer medium, effecting image defects, as a toner image is transferred onto the transfer medium.
Next, the phenomenon that the amount by which electrical charge is given to the residual toner particles is affected by the conditions of the environment in which an image forming apparatus is operated will be described in more detail. The electrical resistance of an electrically conductive brush or the like, which is used as a developer charging member, is greatly affected by the environmental conditions.
Therefore, the value of the bias applied to the electrically conductive brush or the like, as the developer charging member, is kept constant, the toner particles are not given the proper amount of electrical charge.
In other words,
a) In a low humidity/low temperature environment (L/L (15xc2x0 C., 10% RH) environment), the electrical resistance of the developer charging member increases, reducing thereby the amount by which electrical charge is given to the residual toner by the developer charging member (reduction in charging performance). In the case of the first developer charging member, the amount of the force by which the first developer charging member attracts the residual toner, and the amount by which the first developer charging member can hold the residual toner particles, reduce, allowing a larger amount of the residual toner particles to reach and enter the second developer charging member, contact charging member, and developing apparatus, resulting in the generation of ghosts, and/or the contamination of the contact charging member. Further, the second developer charging member falls to give the proper amount of electrical charge to the residual toner particles, which results in the contamination of the contact charging member.
B) In a high humidity/high temperature environment (H/H (30xc2x0 C., 80% RH) environment), the electrical resistance or the developer charging member decreases, allowing an excessive amount of electrical current to flow. Therefore, the amount by which electrical charge is given to the residual toner by the developer charging member is substantially increased (enhancement in charging performance). As a result, not only is the residual toner particles are given a large amount of electrical charge, but also the photoconductive drum is given a large amount of electrical charge; in other words, the first developer charging member injects an excessive amount of positive electrical charge into the photoconductive drum, effecting thereby image defects such as the negative ghosts, brush streaks, etc. Each of these image defects occurs because the photoconductive drum is charged to the polarity (positive polarity) opposite to the polarity to which the photoconductive drum is normally charged. On the contrary, in the case of the second developer charging member, it gives an excessive amount of negative electrical charge. Therefore, as the residual toner particles are charged by the contact charging member on the downstream side of the developer charging members, they fail to be uniformly charged; all the transfer particles are not charged to the predetermined level.
The primary object of the present invention is to provide an image forming apparatus in which all the transfer residual developer particles on the image bearing member are given proper electrical charge.
Another object of the present invention is to provide an image forming apparatus in which all the transfer residual developer particles on the image bearing member are given a proper amount of electrical charge regardless of the conditions of the environment in which the image forming apparatus is used.
Another object of the present invention is to provide an image forming apparatus compatible with a cleaner-less system, that is, a system lacking a dedicated cleaning means.
Another object of the present invention is to provide an image forming apparatus in which all the transfer residual developer particles are efficiently recovered by the developing means.
Another object of the present invention is to provide an image forming apparatus in which the transfer residual developer particles remaining on the image bearing member after image transfer do not cause the image bearing member to be unsatisfactorily charged, and also, do not cause image defects.
Another object of the present invention is to provide an image forming apparatus in which the pattern of the image formed on the image bearing member during the preceding image forming cycle of the image bearing member does not appear on the image bearing member during the following image forming cycle of the image bearing member.
These and other objects, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.